Method and apparatus for detecting battery capacity

ABSTRACT

A method and apparatus for detecting the capacity of a battery wherein a voltage method of measuring the voltage of the battery, to calculate the capacity of the battery based on the correlation between the voltage and the capacity of the battery, is switched to a current integrating method of integrating the current magnitude of the battery with respect to time, to calculate the capacity of the secondary battery, and vice versa, with a pre-set current magnitude as a threshold value, in order to detect the capacity of the battery. By selectively using the voltage method and the current integrating method depending on the current magnitude of the battery, the capacity of the battery can be calculated with greater accuracy.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for detecting the capacity ofa battery loaded on a piece of electronic equipment, such as a portablepersonal computer, for furnishing the power. The present invention alsorelates to an associated battery pack and to an associated electronicequipment system.

2. Description of the Prior Art

In order to know the capacity (residual capacity) of a battery, such asa lithium ion battery, it is generally practiced to estimate theresidual capacity from a terminal voltage of the battery or to integratethe efflux current to estimate the residual capacity.

As a method for detecting the capacity (residual capacity) of suchbattery, there are currently proposed a voltage method and a currentintegrating method. The voltage method measures the voltage of thebattery to calculate the capacity of the battery based on thecorrelation between the voltage and the capacity of the battery. Thecurrent integrating method integrates the current of the battery withrespect to time to calculate the capacity of the battery. In the voltagemethod, the capacity calculation accuracy is high when the current ofthe battery is small, conversely, in the current integrating method(coulomb method), the capacity calculation accuracy is high when thecurrent of the battery is large. Since the voltage method is based onthe voltage, there is no integrating error, but there is a direct error.Since the current integrating method integrates and updates the currentwith respect to a reference value, the integration error is significant,though the direct error is small.

The voltage method calculates the capacity from the correlation betweenthe capacity and the terminal-to-terminal voltage of the battery (cellvoltage). Since the battery (cell) has an internal resistance and ahence A the terminal voltage is fluctuated depending on the flowingcurrent, correction is applied based on the current multiplied by theinner battery voltage. If the current is enlarged, the amount ofcorrection becomes larger to increase the error.

The current integrating method integrates the current with respect totime to find the electrical quantity Ah. For improving the precision,current measurement accuracy needs to be raised. For this reason, anoperational amplifier or an analog-to-digital (A/D) converter ofextremely high precision is used. Nevertheless, there is producedsignificant error if charging/discharging is repeated over an extendedtime interval, or if the current is of a small magnitude, so thatcontrivances such as calibration are occasionally required such as atthe time of charging the battery to its fill capacity. If thecalibration timing is lost, significant errors are inevitably produced.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems, an object of the presentinvention is to provide a battery capacity detection method, a batterypack and an electronic equipment system, in which the voltage method andthe current integration method are selectively used depending on themagnitude of the current through the battery to raise the calculationaccuracy of the capacity of the battery (residual capacity).

For solving the above problem, the present invention provides a methodfor detecting the capacity of a battery in which a voltage method ofmeasuring the voltage of the battery to calculate the capacity of thebattery, based on the correlation between the voltage and the capacityof the battery, is switched to a current integrating method ofintegrating the current magnitude of the battery with respect to time,to such is done calculate the capacity of the battery, and vice versa,with a pre-set current magnitude as a threshold value, in order todetect the capacity of the battery.

The present invention also provides a method for detecting the capacityof a battery in which a voltage method of measuring the voltage of thesecondary battery, to calculate the capacity of the battery based on thecorrelation between the voltage and the capacity of the battery, isswitched to a current integrating method of integrating the currentmagnitude of the battery with respect to time to calculate the capacityof the battery, and vice versa, with a pre-set magnitude of voltage dropas a threshold value, in order to detect the capacity of the battery.

With the capacity detection method according to the present invention,the capacity of a battery can be calculated with greater accuracy byswitching between the voltage method and the current integrating methodresponsive to a pre-set current magnitude or to a pre-set magnitude ofthe voltage drop.

More specifically, the present invention provides a battery pack havingthe function of detecting the capacity of a battery, wherein the batterypack includes voltage detection means for detecting the voltage of thebattery to calculate the capacity of the battery based on; correlationbetween the voltage and the capacity of the battery, current detectionmeans for detecting the current magnitude of the battery to calculatethe capacity of the battery by integrating the current magnitude of thebattery with respect to time, and control means for switching betweenthe operation of calculating the capacity of the battery based on thecorrelation between the voltage and the capacity of the batteryresponsive to a pre-set current magnitude and the operation ofcalculating the capacity of the battery by integrating the currentmagnitude of the battery with respect to time.

With the pack of the battery according to the present invention, thecontrol means is responsive to the pre-set current magnitude or to thepre-set magnitude of the voltage drop to switch between the operation ofcalculating the capacity of the battery based on the correlation betweenthe voltage and the capacity of the battery and the operation ofcalculating the capacity of the battery by integrating the currentmagnitude of the secondary battery with respect to time to improve theaccuracy in capacity calculations.

The electronic equipment system of the present invention resides in theabove-described battery pack loaded on a piece of electronic equipment,such as a personal computer, in which the calculation accuracy of thebattery can be improved by switching between the voltage method and thecurrent integrating method during the capacity calculations.

Additional features and advantages of the present invention aredescribed in, and will be apparent from, the Detailed Description of thePreferred Embodiments and the Description of the Drawings.

BREIF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electronic equipment system including of a battery packfor realizing the battery capacity detection method or the presentinvention and a personal computer, given as an example of an associatedpiece of electronic equipment.

FIG. 2 shows a typical relation between the cell voltage of the batteryand the capacity (%) in the voltage method.

FIG. 3 shows an example of the current integration method.

FIG. 4 shows illustrative detection of the current capacity in thecurrent integration method shown in FIG. 3.

FIG. 5 shows an example of setting a current magnitude as a switchingpoint.

FIG. 6 shows an example of setting voltage drop magnitude as a switchingpoint.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a battery pack 20 (pack of a battery E) for carrying outthe capacity detection method for a battery according to the presentinvention and a personal computer 30 as typical of a piece of electronicequipment on which is loaded the battery pack 20. This personal computer30 is, for example, a portable personal computer, on which battery pack20 can be loaded and which is actuated by the power fed from thisbattery pack 20.

To a bus line BUS of a main central processing unit (CPU) of thepersonal computer 30 are connected a variety of peripheral devices 37, amemory 38, such as a read-only memory (ROM) or a random-access memory(RAM), and a communication LSI 39, etc. A power control circuit 32 isprovided with a power source switch 33 to perform power on/off control.In addition, commercial ac power from a power source plug 35 is fed tothe power control circuit 32 via an ac adapter 34 to supply the powerfrom the battery pack 20 via +terminal 12 and a −terminal 13 for batteryconnection as later explained. The charging current is fed via terminals12, 13 to the battery pack 20.

Turning to FIG. 1 for explanation of the structure of the battery pack20, the battery pack 20 includes a voltage detection circuit 4, acurrent detection circuit 3, and a controller 1. The controller 1includes a micro-computer 11, a storage unit 5 and a read-only memory(ROM) 10. The micro-computer 11 includes a communication terminal 9.

The voltage detection circuit 4, detects the voltage of the battery E inorder to calculate the capacity of the battery E based on thecorrelation between the voltage of the battery E and the capacity. Thebattery E is made up of four battery cells 41 a, 41 b, 41 c, 41 d and,for measuring the voltages of these battery cells 41 a, 41 b, 41 c, 41d, a multiplexer 42 and an operational amplifier 43 are provided in thevoltage detection circuit 4. By battery cell selection control signalsbeing fed from the micro-computer 11 in the controller 1 to themultiplexer 42, the four battery cells 41 a, 41 b, 41 c, 41 d aresequentially selected by multiplexer 42, which then sends the terminalvoltages of the four batteries to an operational amplifier 43. Thevoltage detection signals from the operational amplifier 43 are sent tothe controller 1 and analog-to-digital (A/D) converted in the controller1 so that the battery cell terminal voltage is retrieved as digitalvalues by the micro-computer 11.

The current detection circuit 3, detects the current magnitude of thebattery E in order to integrate the current magnitude of the battery Ewith respect to time to calculate the capacity of the battery E. Thiscurrent detection circuit 3 includes a resistor 44 for currentmeasurement which is connected to, for example, the minus side of thebattery E and an operational amplifier 45 for detecting the voltagecorresponding to the current flowing in the resistor 44. The currentdetection signal from the operational amplifier 45 is sent to thecontroller 1 where it is analog-to-digital (A/D) converted so that themeasured current value is retrieved as a digital value in themicro-computer 11.

To the multiplexer 42 and the operational amplifiers 43, 45, power isfed from the plus side of the battery cell set via a power saving switch46. This switch 46 is tuned on/off by the power save signals suppliedfrom the micro-computer 11. A control switch 2 for controlling thecharging/discharging on/off is connected across the +terminal 7 of thebattery pack 20 (plus battery terminal) and the plus side of the batteryE. This control switch 2 includes a series connection of an FET 21, as acharging switching element, and an FET 33, as a discharging switchingelement. A pair of diodes 33, 34 are connected in parallel with theseFETs 21, 22. The FET 21 for charging is on/off controlled by the driver25 driven by the control signals supplied from the micro-computer 11.The FET 22 for discharging is on/off controlled by the driver 26, whichis driven by control signals supplied from the micro-computer 11 of thecontroller 1.

The controller 1, having the micro-computer 11, is configured to switchbetween the operation of calculating the capacity of the battery E,based on the relation between the voltage and the capacity of thebattery E, using a prescribed current magnitude or a pre-set voltagedrop value as a threshold or switching point, and the operation ofintegrating the current magnitude of the battery E with respect to timeto calculate the capacity of the battery E. The relation between thecell voltage detected by the voltage detection circuit 4 and thecapacity (%) of the battery E is shown in FIG. 2, in which the cellvoltage CE is plotted on the ordinate and the current capacity to fullcapacity ratio, expressed in % (capacity %) is plotted on the abscissa.The point A in FIG. 2 denotes the time point of start of discharge. Thecell voltage CE is decreased substantially linearly, while the capacity% of the battery E is also decreased substantially linearly. Thereexists a voltage value CE1 at which the cell voltage is decreasedabruptly.

The controller 1, in particular the micro-computer 11, calculates thecapacity % based on a voltage detection value DE of the cell voltage CEobtained by the voltage detection circuit 4 based in turn on therelation between the cell voltage CE and the capacity % of FIG. 2.

The battery pack 20 monitors the state of the battery E, such as itsvoltage, charging/discharging current or residual capacity, to exchangedata with a charger, not shown, or a load, such as the personal computer30, by way of having the communication. To this end, the battery pack 20has, enclosed therein, the controller 1 having a cell monitoring andcontrolling micro-computer 11. It is possible with this battery pack 20to display the states of the battery E, sent thereto from themicro-computer 11 over the communication terminal 9, on a displayprovided on the load or on the charger, to advise the user of suchstates.

The battery E, enclosed in the battery pack 20, includes a set of fourlithium-ion-based battery cells 41 a, 41 b, 41 d and 41 d, such abattery E could be a Nicd battery as well. The +terminal of the batteryE is connected via the switch 2 to a +terminal 7 of the pack (package ofthe battery pack 20), while its −terminal is connected via currentdetection circuit 3 to a −terminal 8 (GND terminal). The battery pack 20is loaded in a battery housing section, not shown, provided in thepersonal computer 30, whereby the +terminal 7 of the pack side iselectrically connected to the +terminal 12 of the personal computer 30and the −terminal 8 on the pack side is electrically connected to the−terminal 13 of the personal computer 30. When the battery pack 20 ischarged, the charging current also flows through the +terminal 7 and the−terminal 8.

The micro-computer 11 of the controller 1 is, for example, a centralprocessing unit (CPU), and is configured for periodically receiving theoutput of the current detection circuit 3 or the voltage detectioncircuit 4 to recognize the current flowing through the battery E(charging current and discharging current) or the voltage of the batteryE. The micro-computer 11 controls the normally-on control switch 2,based on the voltage or current, to turn off the control switch 2 tointerrupt the current (charging current and discharging current) toprohibit overcharging, over discharging or excess current.

The micro-computer 11 finds the current residual capacity of the batteryE, based on the voltage of the battery E recognized as described above.The micro-computer 11 also finds the integrated capacity for thecharging capacity, based on the current capacity found as describedabove.

The micro-computer 11 is connected to the communication terminal 9 ofthe pack. This communication terminal 9 is electrically connected to acommunication terminal 14 of the personal computer 30 when the batterypack 20 is loaded on the personal computer 30. The communicationterminal 14 of the personal computer 30 is connected to thecommunication LSI 39 so that, when the battery pack 20 is loaded on thepersonal computer 30, communication occurs between the micro-computer 11of the battery pack 20 and the communication LSI 39 of the personalcomputer 30, via communication terminals 9, 14, in accordance with apre-set communication sequence.

Specifically, the controller 1, in particular the micro-computer 11,performs preset processing responsive to the data (commands etc) sentthereto from the communication terminal 14 of the personal computer 30,or transmits the cell voltage, charging/discharging current, residualcapacity of the battery E or the integrated capacity, to thecommunication terminal 14 of the personal computer 30 via communicationterminal 9. The battery E is for example, a lithium ion cell. Thevoltage of the battery E, termed an open voltage or a cell voltage, isrelated with the residual capacity in a manner as shown in FIG. 2 suchthat, if the cell voltage is found, the residual capacity, expressed interms of % relative to the full capacity, can be found. Thus, themicro-computer 11 finds the residual capacity of the battery B, such asthe lithium-ion-cell, based on the cell voltage, as described above.

The control switch 2 shown in FIG. 1 operates under control of themicro-computer 11 to turn the charging/discharging current on or off.The current detection circuit 3 detects the current flowing therein,that is the discharging current of the battery E, as well as thecharging current to the battery E, to send the detected results to themicro-computer 11. The storage unit 5 includes a register for storage ofthe integrated capacity. A display unit 6 is for example, a liquidcrystal display which displays the information such as the integratedcapacity under control by the micro-computer 11.

In the ROM (read-only memory) 11 are stored programs or data necessaryfor the operation of the micro-computer 11. That is, the micro-computer11 refers to the data stored in the ROM 10 as the occasion may demand toexecute the programs stored in the ROM 10 to perform a variety ofoperations.

When the parallel flat plate 20 is loaded in normal fashion on thepersonal computer 30, the +terminal 7, −terminal 8 and the communicationterminal 9 are electrically connected to the terminals 12 to 14 of thepersonal computer 30, respectively. The personal computer 30 operateswith the battery pack 20 as the picture signals, with the dischargecurrent of the battery E flowing through the +terminals 7, 12, personalcomputer 30 and through the −terminals 13, 8.

In the battery pack 20, the current detection circuit 3 or the voltagedetection circuit 4 detects the current (charging or dischargingcurrent) flowing in the battery E, or the cell voltage, respectively.These current and voltage values are periodically received in themicro-computer 11. The micro-computer 11 verifies, on the basis of thesecurrent or voltage values, whether or not the battery E is in theovercharged or over discharged state or in the overcurrent state. If thebattery E is in the overcharged or over discharged state, the controlswitch 2 is turned off to break the current (charging or dischargingcurrent).

The micro-computer 11 calculates the current capacity (residual capacityof the battery E), based on the cell voltage of the battery E, andfurther calculates the integrated capacity, as reference is had to aregister unit 5, based on the calculated current capacity. Themicro-computer 11 transmits the integrated capacity, calculated asdescribed above, the current value supplied from the current detectioncircuit 3, and the voltage value supplied from the voltage detectioncircuit 4, via the communication terminal 9, responsive to the requestfrom the personal computer 30. The micro-computer 11 also sends thecalculated integrated capacity to the display unit 6 for displaythereon.

A preferred typical example of the current integrating method isexplained with reference to FIGS. 3 and 4. In the current integratingmethod, in which the flowing current is integrated to find Ah(ampere/hour), the current magnitude needs to be measured accurately.For example, if the maximum current for measurement is 10 A and theminimum measurable current is 1 mA, the magnitude which represents 10 A,referred to 1 mA as the minimum unit, is 10000, such that, forrepresenting 10000 decimal, 14 bits (2¹⁴=16384) are required. Therefore,for representing data of measured values in the central processing unit(CPU) of the micro-computer 11 of FIG. 1, 14 bits are theoreticallyrequired. That is, a 14-bit analog-to-digital (A/D) converter isrequired.

If, in integrating the 14-bit data, the integration spacing is decreasedexcessively, the memory capacity required for integration is increased.Thus, the minimum integration resolution is set to find a practicalintegration spacing.

If, for example, 1 mA is set as the resolution (minimum integrationresolution), since the maximum current is 10 A, the minimum integrationspacing is

1[mAh]/10[A]=3600[mAsec]/10000[mA]=0.36[sec]

or 0.36 sec.

If the capacity of the battery is 4800 mAh, since 13 bits binary(2¹³=8192) are required for representing 4800 decimal, the number ofbits required for representing the integrated data value is 14 bits+13bits=27 bits, which may be said to be a practical level of the requiredmemory capacity.

As another example of converting the analog current magnitude formeasuring the integrated current value in the current integratingmethod, there is such a method which uses an analog integrator 100 incombination, as shown in an illustrative circuit in FIG. 3. Thiscircuit, gives an example of an integrator for carrying out the currentintegrating method and the concept of integration. It is noted that thecircuit of FIG. 3 stands for one of the charging direction or thedischarging direction. Two of the circuits shown in FIG. 3 are requiredfor doing both charging and discharging.

The analog integrator 100 has an input terminal 101 and a reset switch102. An output of the analog integrator 100 is connected to a voltagecomparator 103. An output pulse of the voltage comparator 103 to beintegrated is 1 mAh and is used for actuating a reset switch 102.

In the present example of the current integrating method, an outputpulse 104 shown in FIG. 3 is outputted each time the analog integrator100 overflows that is, each time the output reaches the level L andoutput pulses 104 are integrated, as shown in FIG. 4. That is, the timewhen the analog integrator 100 overflows, or the time when the output ofthe analog integrator 100 reaches the level L, is equivalent to the timepoint 1 mAh is measured. Thus, the current magnitude in mAh can beobtained by integrating the pulses from the voltage comparator.

Since the integration is a continuous operation, it is unnecessary totake the resolution into account. However, it is necessary to takeaccount of the offset or drift proper to the analog integrator 100 whichtends to deteriorate calculation accuracy.

Meanwhile, the aforementioned accuracy of the order of 14 bits isrealized in certain ones of the currently marketed A/D converters. Theaforementioned use of the 14-bit A/D converter is for theoreticalconsiderations only and the precision realized in practical applicationis not so high as is contemplated in the present example. If currentmeasurement is achieved to this order of accuracy, the current of thebattery below 1 mA cannot be accommodated. There are occasions whereinno battery is used or the current cannot be measured based on theself-discharging of the battery. This point is taken into considerationin the present embodiment by using the voltage method in the smallcurrent range in place of the current integrating method so that theaforementioned high accuracy is not required. For example. A 10-bit A/Dconverter suffices for practical application.

Referring to FIGS. 5 and 6, an instance of switching between the voltagemethod and the current integrating method based on a pre-set switchingpoint is explained. In the present embodiment, the current integratingmethod and the voltage method are used for the larger and smallermagnitudes of the battery E for highly accurate calculation of thecapacity of the battery E, respectively. This enables the calculationaccuracy of the battery E to be improved even if the precision of thedevice used in the voltage detection circuit 4 for the voltage method orin the current detection circuit 3 for the current integrating method isnot that high.

First, the system of setting a predetermined current value as athreshold value as the switching point (see FIG. 5) is explained. Inthis case, the threshold value as the switching point is set by thecurrent value itself. Since the current value (threshold value) isfixed, the current measurement accuracy (error) of the currentintegrating method can be calculated easily. For example, the internalresistance of the battery E is increased with lowering in temperaturesuch that, at −10° C., the internal resistance is occasionally severaltimes that at ambient temperature. If, for example, the internalresistance is increased by a factor of four, is 250 mΩ at ambienttemperature and 400 mA is set as the current magnitude corresponding tothe switching point, the voltage drop of the battery E, which is 0.1V atambient temperature, is as low as 0.4V at lower temperatures. If theaccuracy for the voltage drop of 0.1V is good and that for the voltagedrop of 0.4V is stringent, the current magnitude corresponding to theswitching point needs to be set to a lower current magnitude, such as100 mA. This renders the current measurement accuracy more stringent.

In the present method, if the current magnitude of the battery E exceedsa predetermined current magnitude, as set as the switching point(threshold value), the capacity integration of the capacity of thebattery E is switched to the current integrating method. Conversely, ifthe current magnitude of the battery E is lower than the predeterminedcurrent magnitude (threshold value) as the switching point, the capacityintegration of the capacity of the battery E is switched to the voltagemethod.

Next, the case of setting a predetermined voltage drop value of thebattery E as the threshold value as this switching point (see FIG. 6) isexplained. If the predetermined voltage drop value of the battery E isset as the threshold value as this switching point, the precisioncondition for the voltage method is easy to set, because the magnitudeof the voltage drop (threshold value) is constant. If, for example, themagnitude of the voltage drop as the switching point is set to 0.1V, thecurrent is 400 mA and 100 mA at ambient temperature and at lowertemperature, respectively.

If the magnitude of the voltage drop of the battery E is larger than thethreshold value as the switching point (with the predetermined magnitudeof the voltage drop being, for example, 0.1V), more current is flowing,so that the current integrating method is used for detecting thecapacity of the battery E. If the magnitude of the voltage drop of thebattery E is smaller than the predetermined magnitude of the voltagedrop of the switching point (threshold value), the voltage method isused as the method for detecting the capacity of the battery E.

The above-mentioned current magnitude or the magnitude of the voltagedrop as the switching point is given merely as illustration. It is,however, possible to switch between the current integrating method andthe voltage method only gradually or stepwise instead of completelyswitching between the current integrating method and the voltage method.It suffices then to provide a width to each of the preset currentmagnitude or magnitude of the voltage drop for switching and to switchgradually between cell capacity detection by the current integratingmethod for the larger current magnitude of the battery and that by thevoltage method for the smaller current magnitude. Alternatively, pluralthreshold values are provided as the preset current magnitude or themagnitude of the voltage drop to switch between cell voltage detectionby the voltage method and that by the current integrating methodstepwise responsive to plural threshold values.

If, due to abrupt changes in the current magnitude or switching at asole threshold value, switching between the current integrating methodand the voltage method has occurred at one time, it is desirable to makeprocessing for gradually changing the integrated capacity value. If theaccuracy error is zero in the voltage method or in the currentintegrating method, this processing is not required. Since there existsa slight difference between the two systems, this processing is to beperformed.

In the current integrating method, the cell capacity is calculated basedon the integrated magnitude of the current that has flowed (in Ah orcoulombs). In the voltage method, the cell capacity is determined by apre-formulated table showing the cell voltage potted against thecapacity (%) (see FIG. 2). In the current integrating method, thecurrent magnitude obtained is subtracted from the full charged capacityof the battery E to find the residual capacity, conversely, in thevoltage method, the current magnitude obtained is multiplied with thefull charged capacity of the battery E to find the residual capacity.Thus, calculation errors, conversion errors or table errors representinfluencing factors. Meanwhile, integration in the current integratingmethod is by integration with the capacity value obtained with thevoltage method as the base point.

That is, when switching from the voltage method to the currentintegrating method, the ultimate cell capacity is found by integratingthe current magnitude calculated by the current integrating method tothe current cell capacity corresponding to the current cell capacity(cell capacity at the switching point), as found by the voltage method,as the base point. Conversely, in switching from the current integratingmethod to the voltage method, there are occasions when an error isproduced between the current cell capacity (at the switching point) asfound by current integration and the cell capacity directly afterswitching as found on the basis of the correlation between the voltageand the cell capacity. It is therefore desirable to take the errorbetween the cell capacities into account to switch gradually from thecell capacity value as detected by the current integrating methoddirectly before switching to the cell capacity value as detected by thevoltage method directly after the switching.

It is also possible to estimate the full charging capacity ordeterioration from the coulomb values between pre-set voltages or toconstruct the table of the voltage method from the highly accurateinformation of the current magnitude obtained by the current integratingmethod in case the current magnitude is constant and large such asduring charging. The voltage method exploits the voltage-currentcharacteristics of the lithium ion secondary battery to estimate thecapacity % from the voltage of the cell terminals, as described above.If the magnitude of the voltage drop is significant, such as when thecurrent is large, the voltage method is low in capacity measurementaccuracy of the battery E because of overlap of various errors.Conversely, with the current integrating method, the integration erroris large if the current is small, whereas, if the voltage fluctuationsare large, the capacity accuracy is low.

With the cell capacity detection method according to the presentinvention, it is possible to obviate the deficiencies of the voltagemethod and the current integrating method reciprocally to detect thecapacity of the battery E highly accurately by switching between thesetwo methods at a preset switching point. The switching point may be thecurrent magnitude or the magnitude of the voltage drop, as describedabove.

The present invention also may be applied to the cell pack (batterypack) to which the above-described cell capacity detection method isapplied, and to an electronic equipment system which includes the cellpack and the electronic equipment for loading the cell pack.

With the above-described battery capacity detection method, battery packand the electronic equipment system, of the present invention, thecalculation accuracy of the capacity of the battery (residual capacity)can be increased by selectively using the voltage method and currentintegrating method depending on the current magnitude in the battery.

Although the present invention has been described with reference tospecific embodiments, those of skill in the art will recognize thatchanges may be made thereto without departing from the spirit and scopeof the invention as set forth in the hereafter appended claims.

What is claimed is:
 1. A method for detecting battery capacity of abattery, comprising the steps of: detecting the battery capacity by avoltage method, the voltage method including measuring a voltage of thebattery and calculating the battery capacity based on a correlationbetween the voltage and the battery capacity; detecting the batterycapacity by a current integrating method, the current integrating methodincluding integrating a current magnitude of the battery with respect totime to calculate the battery capacity; and switching between thevoltage method and the current integrating method based on a pre-setcurrent magnitude to detect the battery capacity.
 2. A method fordetecting battery capacity of a battery as claimed in claim 1, furthercomprising the steps of: setting a threshold value as the pre-setcurrent magnitude; detecting if the current magnitude of the battery islarger than the threshold value; detecting the battery capacity by thecurrent integrating method if the current magnitude of the battery islarger than the threshold value; and detecting the battery capacity bythe voltage method if the current magnitude of the battery is not largerthan the threshold value.
 3. A method for detecting battery capacity ofa battery as claimed in claim 1, further comprising the step of:according a certain width to the pre-set current magnitude to effect agradual switching between the current integrating method for largecurrent in the battery and the voltage method for small current in thebattery.
 4. A method for detecting battery capacity of a battery asclaimed in claim 1, further comprising the step of: setting a pluralityof threshold values as the pre-set current magnitude to effect stepwiseswitching between the current integrating method for large current inthe battery and the voltage method for small current in the battery. 5.A method for detecting battery capacity of a battery as claimed in claim1, wherein, in switching from the voltage method to the currentintegrating method, the battery capacity is found by integrating thecurrent magnitude as detected by the current integrating method with thebattery capacity as detected by the voltage method as a base point.
 6. Amethod for detecting battery capacity of a battery as claimed in claim1, wherein, in switching from the current integrating method to thevoltage method, an ultimate battery capacity is found by graduallyswitching between the battery capacity as found by the currentintegrating method and the battery capacity as found by the voltagemethod.
 7. A method for detecting battery capacity of a battery,comprising the steps of: detecting the battery capacity by a voltagemethod, the voltage method including measuring a voltage of the batteryand calculating the battery capacity based on a correlation between thevoltage and the battery capacity; detecting the battery capacity by acurrent integrating method, the current integrating method includingintegrating a current magnitude of the battery with respect to time tocalculate the battery capacity; and switching between the voltage methodand the current integrating method based on a pre-set voltage drop ofthe battery to detect the battery capacity.
 8. A method for detectingbattery capacity of a battery, as claimed in claim 7, further comprisingthe steps of: setting a threshold value as the pre-set voltage drop;detecting if the current magnitude of the battery is larger than thethreshold value; detecting the battery capacity by the currentintegrating method if the current magnitude of the battery is largerthan the threshold value; and detecting the battery capacity by thevoltage method if the current magnitude of the battery is not largerthan the threshold value.
 9. A method for detecting battery capacity ofa battery as claimed in claim 7, further comprising the step of:according a certain width to the pre-set voltage drop to effect gradualswitching between the current integrating method for a large voltagedrop in the battery and the voltage method for a small voltage drop inthe battery.
 10. A method for detecting battery capacity of a battery,as claimed in claim 7 further comprising the step of: setting aplurality of threshold values as the pre-set voltage drop to effectstepwise switching between the current integrating method for a largevoltage drop in the battery and the voltage method for a small voltagedrop in the battery.
 11. A method for detecting battery capacity of abattery as claimed in claim 7, wherein, in switching from the voltagemethod to the current integrating method, the battery capacity is foundby integrating the current magnitude as detected by the currentintegrating method with the battery capacity as detected by the voltagemethod as a base point.
 12. A method for detecting battery capacity of abattery as claimed in claim 7, wherein, in switching from the currentintegrating method to the voltage method, an ultimate battery capacityis found by gradually switching between the battery capacity as found bythe current integrating method and the battery capacity as found by thevoltage method.
 13. A battery pack for detecting battery capacity of abattery, comprising: voltage detection means for detecting a voltage ofthe battery to calculate the battery capacity based on a correlationbetween the voltage and the battery capacity; current detection meansfor detecting a current magnitude of the battery to calculate thebattery capacity by integrating the current magnitude of the batterywith respect to time; and control means for switching betweencalculating the battery capacity based on the correlation between thevoltage and the battery capacity responsive to a pre-set currentmagnitude, and calculating the battery capacity by integrating thecurrent magnitude of the battery with respect to time.
 14. A batterypack as claimed in claim 13, wherein a threshold value is set as thepre-set current magnitude, and wherein the battery capacity iscalculated by integrating the current magnitude of the battery withrespect to time if the current magnitude is larger than the thresholdvalue, and wherein the battery capacity is calculated based on thecorrelation between the voltage and the battery capacity if the currentmagnitude is smaller than the threshold value.
 15. A battery pack asclaimed in claim 13, wherein a certain width is accorded to the pre-setcurrent magnitude to effect gradual switching between the currentmagnitude integration in case of a large current in the battery and thecorrelation between the voltage and the battery capacity for a smallercurrent in the battery.
 16. A battery pack as claimed in claim 13,wherein a plurality of threshold values are set as the pre-set currentmagnitude to effect stepwise switching between the current magnitudeintegration for a large current in the battery and the correlationbetween the voltage and the battery capacity for a small current in thebattery.
 17. A battery pack as claimed in claim 13, wherein, inswitching from the correlation between the voltage and the batterycapacity to the current magnitude integration, the battery capacity isfound by integrating the current magnitude as detected by the currentmagnitude integration with the battery capacity as detected based on thecorrelation between the voltage and the battery capacity as a basepoint.
 18. A battery pack as claimed in claim 13, wherein, in switchingfrom the current magnitude integration to the correlation between thevoltage and the battery capacity, an ultimate battery capacity is foundby gradual transition from the current magnitude integration to thecorrelation between the voltage and the battery capacity.
 19. A batterypack for detecting battery capacity of a battery, comprising: voltagedetection means for detecting a voltage of the battery to calculate thebattery capacity based on a correlation between the voltage and thebattery capacity; current detection means for detecting a currentmagnitude of the battery to calculate the battery capacity byintegrating the current magnitude of the battery with respect to time;and control means for switching between calculating the battery capacitybased on the correlation between the voltage and the battery capacityresponsive to a pre-set voltage drop of the battery, and calculating thebattery capacity by integrating the current magnitude of the batterywith respect to time.
 20. A battery pack as claimed in claim 19, whereina threshold value is set as the pre-set voltage drop, and wherein thebattery capacity is calculated by integrating the current magnitude ofthe battery with respect to time if the voltage drop is smaller than thethreshold value, and wherein the battery capacity is calculated based onthe correlation between the voltage and the battery capacity if thevoltage drop is larger than the threshold value.
 21. A battery pack asclaimed in claim 19, wherein, a certain width is accorded to the pre-setvoltage drop to effect gradual switching between the current magnitudeintegration in case of a large voltage drop of the battery and thecorrelation between the voltage and the battery capacity for a smallervoltage drop in the battery.
 22. A battery pack as claimed in claim 19,wherein a plurality of threshold values are set as the pre-set voltagedrop to effect stepwise switching between the current magnitudeintegration for a large voltage drop in the battery and the correlationbetween the voltage and the battery capacity for a small voltage drop inthe battery.
 23. A battery pack as claimed in claim 19, wherein, inswitching from the correlation between the voltage and the batterycapacity to the current magnitude integration, the battery capacity isfound by integrating the current magnitude as detected by the currentmagnitude integration with the battery capacity as detected based on thecorrelation between the voltage and the battery capacity as a basepoint.
 24. A battery pack as claimed in claim 19, wherein, in switchingfrom the current magnitude integration to the correlation between thevoltage and the battery capacity, an ultimate battery capacity is foundby gradual transition from the current magnitude integration to thecorrelation between the voltage and the battery capacity.
 25. Anelectronic equipment system, comprising: a battery pack for detectingbattery capacity of a battery; an electronic piece of equipment forremovably mounting the battery pack via an electrically connectingportion wherein the electronic piece of equipment is fed with power fromthe battery pack; voltage detection means in the battery pack fordetecting a voltage of the battery to calculate the battery capacity ofthe battery based on a correlation between the voltage and the batterycapacity; current detection means in the battery pack for detecting acurrent magnitude of the battery to calculate the battery capacity ofthe battery by integrating the current magnitude with respect to time;and control means in the battery pack for switching, in response to apre-set current magnitude, between the operation of calculating thebattery capacity of the battery based on the correlation between thevoltage and the battery capacity and the operation of integrating thecurrent magnitude of the battery with respect to time to calculate thebattery capacity of the battery.
 26. An electronic equipment system asclaimed in claim 25, wherein a threshold value is set as the pre-setcurrent magnitude, and wherein the battery capacity is calculated byintegrating the current magnitude of the battery with respect to time ifthe current magnitude is larger than the threshold value, and whereinthe battery capacity is calculated based on the correlation between thevoltage and the battery capacity if the current magnitude is smallerthan the threshold value.
 27. An electronic equipment system,comprising: a battery pack for detecting battery capacity of a battery;an electronic piece of equipment for removably mounting the battery packvia an electrically connecting portion wherein the electronic piece ofequipment is fed with power from the battery pack; voltage detectionmeans in the battery pack for detecting a voltage of the battery tocalculate the battery capacity of the battery based on a correlationbetween the voltage and the battery capacity; current detection means inthe battery pack for detecting a current magnitude of the battery tocalculate the battery capacity of the battery by integrating the currentmagnitude with respect to time; and control means in the battery packfor switching, in response to a pre-set voltage drop of the battery,between the operation of calculating the battery capacity of the batterybased on the correlation between the voltage and the battery capacityand the operation of integrating the current magnitude of the batterywith respect to time to calculate the battery capacity of the battery.28. An electronic equipment system as claimed in claim 27, wherein athreshold value is set as the pre-set voltage drop, and wherein thebattery capacity is calculated by integrating the voltage drop of thebattery with respect to time if the current magnitude is smaller thanthe threshold value, and wherein the battery capacity is calculatedbased on the correlation between the voltage and the battery capacity ifthe voltage drop is larger than the threshold value.